Medical devices can be implanted in a body to perform tasks including monitoring, detecting, or sensing physiological information in or otherwise associated with the body, diagnosing a physiological condition or disease, treating or providing a therapy for a physiological condition or disease, or restoring or otherwise altering the function of an organ or a tissue. Examples of an implantable medical device can include a cardiac rhythm management device, such as a pacemaker, a cardiac resynchronization therapy device, a cardioverter or defibrillator, a neurological stimulator, a neuromuscular stimulator, or a drug delivery system. In certain examples, the implantable medical device can include a telemetry circuit and an antenna, coupled to the telemetry circuit, the combination of which can be configured to provide wireless communication between the implantable medical device and an external device, e.g., to send information (such as physiological or other information) from the implantable medical device to the external device, or to receive information (e.g., such as programming instructions) at the implantable medical device from the external device.
Magnetic coupling can be used to provide short-range (e.g., a few centimeters) communication between an implantable medical device implanted in a body and an external device, or between an implantable medical device outside of the body and an external device. However, magnetic coupling communication largely relies on near-field radiation, where the field distribution is highly dependent upon the distance from, and orientation of, the antenna, which grossly limits the effective range of wireless communication between the implantable medical device and the external device.
As an alternative to magnetic coupling, or in addition to magnetic coupling, low power radio frequency (RF) communication, having an extended range over magnetic coupling, can be used to provide communication between an implantable medical device and an external device. However, many RF communication channels, bands, or frequencies are limited by governing bodies or other regulatory agencies, such as the Federal Communications Commission (FCC) or other regulatory body. As such, a frequency available for use in one area may not be available for use in another area.
Further, a desired antenna length can be dependent on the specified transmission frequency. For certain frequencies, the desired antenna length can be difficult to contain within a medical device.